Method of examining and testing an electric device such as an integrated or printed circuit

ABSTRACT

This method is based on the radiation-matter interaction in a structure having discontinuities and where electron transport occurs in at least one layer. To this end, a stimulation signal is sent through the device under test and simultaneously at least one source of radiation sends incident radiation towards the surface of the device. The secondary radiation emitted by the device is detected together with the response of the device. The signals are compared, either individually or in combination, with a reference.

BACKGROUND OF THE INVENTION

The invention relates to a non-destructive method of examining andtesting an electric device such as an integrated or printed circuit, ora transducer using the same technologies. Using this method, radiationemitted by the device itself after stimulation is detected by scanning,and electric signals characteristic of the detected radiation areformed. These signals are then compared with reference signals.

A large number of existing micro-electronic devices are made by theintegrated circuit technique, basically by lithography, chemical etching(photogravure) and metal-coating of slices or chips of silicon or ofcompounds III-V and II-VI in the periodic table of the elements. Arraysof photo-detectors are examples of such devices. These examinations canalso be applied to single or multiple-layer printed circuits, chips cutfrom drawn bars of silica or compounds III-V and II-VI in the periodictable of the elements, or to microwave circuits obtained by methodssimilar to those for integrated-circuit chips.

Numerous efforts have already been devoted to automatic examination forfaults in these kinds of electronic devices and transducers. Usually theexamination is made by optical detection of radiation in the visible orinfrared range, generated or reflected by the device. In U.S. Pat. No.3,803,413, for example, an excitation signal (either a direct voltage ora modulated or pulse voltage) is applied to a printed circuit, and theinfrared radiation emitted by the various circuit components is detectedby a scanning mechanism. The radiation density at each point of thecircuit is then compared with a reference so as to detect any faultsassociated with Joule-effect heating of the various metal coatingsmaking up the circuit.

There are also methods using the technique of optical correlation ofimages by analysing the optical radiation reflected by the surface ofthe circuit. This applies particularly to European patent specificationsNos. 66321 and 66694.

The first of the aforementioned processes detects faults mainly relatingto the dimensions of the components. Processes using reflection ofoptical radiation can detect faults in the geometry of the circuit orstresses occurring at the surface, e.g. as a result of distortion of thesubstrate or chip.

On the other hand these inspection processes cannot detect faults in thestructure, inter alia at the interfaces between the various layers orfaults in the substrate. Such faults are not inevitably shown byabnormal infrared radiation during transit of a given electric signal orby simple reflection or scattering of incident radiation.

It has already been proposed to use a scanning electron microscope (SEM)for forming an image by picking up electrons emitted as a result ofelectron scanning. This method has a number of disadvantages,particularly for production tests, since the electric devices under testhave to be placed in a vacuum chamber and covered with a deposited layerof metal, which means that the process cannot be applied rapidly andsystematically to a large production series. This method of irradiationcannot be used for simultaneous electric testing at speeds compatiblewith manufacturing rates. This kind of examination is described interalia in U.S. Pat. No. 4,358,732 and French patent specification No. 2058 756. In the latter document, electric pulse tests are madesimultaneously and in synchronism with electron bombardment. This typeof test cannot locate structural faults but only gives information aboutthe performance of the tested circuit; it is also slow.

GB patent specification No. 2 069 152 also relates to a method oftesting integrated circuits, in which the circuit is supplied with avoltage set to a value slightly higher than the marginal voltage atwhich faults in the circuit cause voltage deviations at the output whena logic test is applied to it. When a beam of radiation scans thesurface, faults are revealed in the characteristic of the resultingphotoelectric current and can thus be detected, but not located. Thisprocess does not comprise displaying faults by using the radiationre-emitted or transmitted by the circuit, and consequently no structuralfaults can be located.

It is known that if discontinuities are present in material excited byincident radiation, there will be physical interaction between theradiation and the discontinuities, resulting in a change in the mode ofexcitation at the interfaces.

When an incident beam strikes a semiconductor or an interface, itproduces electron-hole pairs. If these mobile charge carriers reach orcome from the p-n junction depletion region, they are swept by thejunction potential, producing an external inverse current which issuperposed on the current induced by the external electric test voltage.This current can be collected and amplified to obtain information aboutthe electric state of the junctions through which it passes, orvariations in the junction characteristics.

In addition, when a structure comprising a number of layers havingdifferent electric conductivity, in which electron transport occurs inat least one layer, is excited by incident radiation, the electrontransport stimulates the change in the mode of excitation which occursat the structure layer interfaces and is revealed by induced radiation(called secondary radiation). The signal response is also modified bythe interaction between the radiation and the electric signal producingthe electron transport.

The invention aims to take advantage of these physical phenomena indetecting faults in the structure of electronic devices at theinterfaces (including interfaces between metal coatings and substrate)and some faults inside the substrates themselves.

SUMMARY OF THE INVENTION

To this end, the invention relates to a method of examining and testingan electrical element having a layered structure of material byinitially: passing through the element a predetermined electricalsignal. Simultaneously therewith the element is irradiated withradiation from at least one radiation source of predetermined type andknown characteristics. The combination of this irradiation andelectrical signal stimulates the said element by the irradiationinteracting with the material of the structure of the element throughwhich the signal is passing at discontinuties present in the structure.Scanning the element thereby detects the radiation emitted by theelement owing to the stimulation of the element Electrical signals onthe basis of the detected radiation are then formed and compared withreference signals.

This method can yield a large amount of information in a very shorttime. This information relates not only to the geometry but also tofaults in the structure, particularly the micro-structure. As we shallsee, the sources of excitation by radiation can be chosen inter aliaaccording to the type of faults to be detected; a number of differentsources can be used simultaneously and the secondary radiation inducedby interaction between each form of incident radiation and the structurewhere electron transport occurs, is detected by respective detectorsappropriate to the nature of the induced secondary radiation.

In the stage of the method for processing electric signalscharacteristic of the detected induced radiation, use is made ofalgorithms based on image recognition techniques, which are outside thescope of the invention and will therefore not be discussed in thefollowing description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate schematically, and by way ofexample, the performance of the method according to the invention. Inthe drawings:

FIG. 1 is an explanatory diagram of a unit for examining an electricdevice, for putting the method of the invention into practice;

FIG. 2 is a diagram of a variant of FIG. 1;

FIG. 3 is a diagram of the types of re-emitted secondary radiation;

FIG. 4 is a diagram of the primary excitation zones where secondaryemission is detected, and

FIG. 5 is a block diagram of a examination (test and injection) unit.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates an examination unit comprising an electric device orelement 1 to be tested, at least one radiation source 2 for producingincident radiation r₁ directed towards the surface of the electricdevice element, and at least one detector 3 for picking up secondaryradiation r_(e) emitted as a result of exciting the device 1. Device 1is connected to an electric source 4 adapted to produce a givenelectrical logic signal, which can be a pulse or a modulated signal, ora d.c. voltage. The output of source 4 is also connected to a comparisonmodule 5 for comparing the logic response signal S_(p) produced at theoutput of device 1 with a reference signal S_(r) stored in a memory 6.

The detector or detectors 3 convert the picked-up secondary radiationr_(e) into an electrical response signal that is characteristic of thedetected radiation. This signal is transmitted to an image-formingmodule 7 after scanning of the device 1. Scanning techniques arewell-known and will not be described or illustrated here. It can simplybe stated that scanning of the excitation radiation from source 2 usinga motor-driven rotating mirror can be used with electronic scanning ofthe transducer 3. The images from the image-forming modules 7 aretransmitted to a module 8 which collects the information from comparisonmodule 5 and modules 7 and detects faults in the transmitted images. Theresulting information is finally transmitted to a decision module 9which determines the extent to which the device 1 corresponds to presetstandards.

The method according to the invention relates basically to the combinedexcitation (incident radiation r_(i) and voltage from source 4) ofdevice 1, and more particularly to irradiation of device 1 together withelectron transport generated by the excitation signal S_(e) fromelectric source 4.

The chosen radiation source or sources 2 can easily show various kindsof faults in the structure of device 1 if properly selected. Variousselections are illustrated by way of example, infra. Also, in certaincases, this can be used to influence the response signal S_(p) producedat the output of device 1.

When excitation radiation is applied to an electric device having amulti-layer structure, with electron transport occurring across at leastone layer, the total radiation induced of the combined excitation byincident radiation and by the electron-transport generating signalS_(e), is made up of the reflected part of the incident radiation, thediffracted part thereof, the radiation of the irradiated material itselfinduced in conjunction with the electric excitation, and the radiationof the material itself due to the electric excitation alone.

The most important part of the radiation induced by this combinedexcitation is the secondary radiation of the irradiated material inducedin conjunction with electric excitation. This owes its existence to theinteraction between radiation and material when there arediscontinuities in the structure. An integrated circuit, a printedcircuit, a monolithic transducer or the substrates of such circuits aremainly formed from continuous layers, i.e. discontinuous structureswhere interaction between radiation and material occurs when thestructures are subjected to exciting radiation, due to the change in themode of excitation at the interfaces of the structure. It is found thatthis change is stimulated when at least one layer is traversed byelectron transport due to an excitation signal.

The method according to the invention is also applicable to stacksobtained by epitaxial growth of thin layers of alternatively one and theother of two semiconducting substrates (e.g. GaP or InP, GaAsP orInAsP). These stacks produce strong photo-luminescence when excited. Themethod can also be applied to gallium arsenide (GaAs) or indiumphosphide (InP) semiconductors.

Various kinds of excitation by radiation, and the nature of the faultswhich they are adapted to detect, will now be considered with referenceto examples:

EXAMPLE 1

Electric excitation is combined with irradiation by a pulsed CW ionlaser (0.5 MW/cm² and 2 kA/cm²). This type of excitation allows thedetection of surface contamination, the presence of foreign particles, abroken array, or scratches. Either luminescence in the 0.8-3 μm infraredrange produced by the density of carriers optically generated in theband-to-band radiative recombination of acceptor dopants, ormodifications of the filtered light dispersed in space can be detected.The method can also detect imperfections in the crystalline structurevia luminescence in the 0.8-3 μm infrared range produced by the densityof positive charge carriers in the band-to-band radiative recombinationof acceptor doping agents. This kind of irradiation combined withelectric excitation is also of use for showing errors in alignment orerrors in masks, or discontinuities in the metal coating or brokentracks or conductors. Comparing the detected image with a referenceimage and extracting characteristic features depending on the density ofoptically generated carriers, the current not flowing, or appearing atthe wrong place shows these errors. Finally, this kind of radiation canshow distortions in flat surfaces and discontinuities in metal coating,by comparing the image or the scanning thereof with the characteristicsof a reference image.

EXAMPLE 2

Electric excitation of source 4 can be used simultaneously with a sourceof polarized infrared light or an infrared source for detectingimperfections in the crystal structure, or gaps or scratches in thearray, ruptures in the oxide and etching of the array, by doublerefraction caused by the stresses induced jointly by thermal andelectric excitation. This kind of excitation can also be used to detectirregular diffusion and inter-metallic formations by infra-redabsorption resulting from stimulated radio-chemical reactions. Irregulardiffusion and defective doping can also be detected by transitionrecombinations of O₂ and C in the substrate induced by stimulatedirradiation. Finally, since inter-metallic formations are related todislocation of the core structure, modifications in the structure induceeffects when a thermal stress is applied, thus enabling inter-metallicformations to be detected.

EXAMPLE 3

Electric excitation of source 4 is used together with isotropicmicrowave radiation, more particularly millimeter waves in the 30-90 GHzregion. This kind of combined excitation can be used to measurephoto-induced voltage modifications characteristic of the capacitance.This can be used to trace the variation with time of the electricresponse supplied to module 5 and in the image detected by transducer 3to detect the presence of particles of foreign material, irregulardiffusion, breaks in the metal coating, or migration of electrons. Themillimeter wave detector in the 20-600 GHz region can be aphotoconductive detector containing cadmium/mercury telluridemonocrystals ([Hg_(x-1), Cd_(x) ]Te) in the hot electron mode.Alternatively the infrared radiation produced by the resistance inseries of junctions induced by the excitation currents can be used todetect imperfections in the crystal structure, irregular diffusion anddefective doping. The image produced by modules 7 can be compared withthe characteristic features of the reference image to show breaks intracks or conductors, displaced tracks and surface distortion. This kindof excitation can also detect intermetallic formations, since suchcompounds are associated with dislocation of core structure, andstructural modifications induce effects when a thermal stress isapplied, superposed on the induced acoustic effects. Irregular diffusioncan also be detected by nuclear magnetic resonance (NMR) at themicrowave or infrared level. Finally, intermetallic formations anddefecting doping agents can both be detected by transitionrecombinations of O₂ and C in the substrate, induced by stimulatedirradiation.

EXAMPLE 4

Electric excitation of source 4 is applied together with irradiation byprotons. The density of carriers generated by the protons producesthermal waves which scatter around faults and can show imperfections inthe crystals, broken or scratched arrays, pores in oxides and breakstherein.

EXAMPLE 5

Electric excitation by source 4 is applied together with a pulsedelectron beam, so that the density of carriers generated by the articlesproduces thermal and electric waves which scatter around faults andreveal imperfections in the crystals, broken arrays, or scratches.

EXAMPLE 6

Electric excitation by source 4 is applied together with neutronirradiation. The luminescence induced by the proportion of chargedfaults in free-doublet semiconductors can be used to detectimperfections in the crystals, broken arrays, scratches, and irregulardiffusion.

EXAMPLE 7

Electric excitation by source 4 is applied together with ion irradiationat 60 keV. Guided propagation of incident ions between layers andbetween crystal planes produces secondary electrons for detectingimperfections in crystals, broken arrays, scratches, pores in oxides orirregular diffusion.

Ion sources give information about the sub-surface which is not easilyaccessible to the electron microscope. The only information at a depthis supplied by back-scattering of energetic secondary electrons producedby beams of ions resulting from hard Rutherford diffusion of atomicnuclei into the substrate. Beams of incident ions and emissions ofsecondary electrons can therefore serve as a sensitive probe of theunderlying structure of the target. Cascades of these hard collisions ofatomic nuclei in the substrate convey information towards the surface,inter alia at the electron junctions of the integrated circuit. A lossin the channel current (sensitive to the direction of incident ions) inthe semiconducting layers of the integrated circuit will producehigh-contrast secondary electron images.

As an example of sources of ion bombardment, heavy gallium Ga+ ionshaving 40 keV energy can be used.

EXAMPLE 8

Electric excitation of source 4 is applied together with an X-ray laserbeam. The photo-induced fluorescence stimulated by X-rays can showsurface contamination, particles of foreign materials, cracks orscratching in the array, or faults in alignment and in the masks. Breaksin the metal coating are shown by comparing the image and extractingcharacteristic features depending on the density of optically generatedcarriers, since the current does not flow or appears at the wrong place,or by correlation of images on the basis of characteristic featuresthereof.

EXAMPLE 9

Electric excitation by source 4 is applied together with irradiationfrom a nuclear magnetic resonance (NMR) source, for showingimperfections in crystals, pores in oxides or irregular diffusion.

FIG. 3 is a diagram of the various kinds of radiation which can beemitted by the electric device under test, depending on the variousmethods of irradiation used and listed hereinbefore. In connection withFIG. 3, it is important to note that the image can be detected either byreflection or by transparency through the circuit; transparency e.g. inthe case of 1.15μ or laser beam or soft X-rays. In addition the detectedradiation (reflected or transmitted) comprises wavelengths λ_(d) otherthan the wavelength λ_(e) of the exciting radiation, and the wavelengthsλ_(d) may be either higher or lower than the excitation radiation. Thisfeature has the two important advantages of eliminating interferingreflection at wavelengths λ_(e) and above all, the fact that thesecondary emission is characteristic of transitions at the junctions, sothat the spectrum of the wavelengths λ_(d) of these secondary emissionscan be used to show the nature of the transitions which produced them,more particularly transitions due to structural faults.

It is to be noted that a scanning electron microscope only giveslow-energy of less than 50 eV secondary electrons induced by a primaryelectron beam. In the case of the invention, photons as well aselectrons and secondary ions can be induced, e.g. by an incident laserbeam or a beam of ions or millimeter waves.

The diagram in FIG. 3 also shows as secondary radiation the cathodeluminescence induced by irradiation by electrons or millimeter waves orlaser; such luminescence occurs at certain junctions and also isimproperly etched resin.

The diagram in FIG. 4 shows the depth of the primary excitation zonesproducing secondary emission for each form of induced radiation.

Secondary emission may occur only at certain values of the voltageapplied to the tested circuit, typically for an OR control polarisationvoltage or near the blocking voltage, or voltages chosen in dependenceon the length of junctions, the properties of the substrates, metalcoatings, etc.

The source of radiation 2, and the electric excitation for electricallytesting the circuit and produced by source 4, can be synchronized;stroboscopic illumination can be used to block the spatial distributionof voltages and electron charges in selected logic states, connectedwith electric excitation (in a dynamic and periodic manner).

An opaque mask 12 (FIG. 2) can be placed in front of each detector 3 atthe focal point of a lens thereof, to prevent specular reflections ofthe source 2 on the device 1 from reaching the detectors 3.

The various aforementioned embodiments can be applied under specialtemperature and pressure conditions of the device under test, and in aspecial electro-magnetic or chemical environment. For example, device 1,source or sources 2 and detector or detectors 3 can be placed in asealed chamber 10 (FIG. 2) containing a gas in gaseous or liquid state,such as a neutral gas e.g. N₂ or CO₂ for producing a given chemicalmedium at a controlled temperature. Hot spots may appear and producelocalized evaporation, shown by the formation of bubbles. Chamber 10 canalso serve as a barrier for absorbing electromagnetic radiation outsidethe chamber. It can be supplied with a diffusion or thermo-molecularpump.

It is also proposed to deposit a layer 11 (FIG. 2) on device 1 in orderto enhance certain secondary radiation r_(e) emitted by certain zones ofdevice 1 when simultaneously excited by souces 2 and 4 (FIG. 1). Forexample if layer 11 is made of cholesterol or nematic crystals, detector3 can be a row or array of three detectors in three respective colours.The liquid crystals in layer 11 can be diluted in pentane with 0.05 to2% of lecithin. Cholesterol liquid crystals sensitive to the localelectric or electrostatic field rotate the polarization plane of thelight which they transmit. When hot, the crystals do not rotatepolarized light and therefore appear opaque.

The layer 11 (FIG. 2) covering device 1 may also be a transparentsubstance, e.g. fluorescein diluted in pentane with 0.05 to 2% lecithin,becoming fluorescent when excited by a UV or X-ray laser source as aresult of combined electric and photochemical reactions.

Layer 11 may alternatively be a transparent substance such asfluorescein which becomes luminescent when excited by a source 2 such asa source of low-energy neutrons (2-50 eV) owing to the combination ofchemical and photochemical reactions.

Layer 21 may also be a liquefied or atomized freon or fluorocarbon,sensitive to local heating of device 1.

FIG. 5 is a block diagram of an installation which has been used fortesting integrated circuits IC.

The irradiation part of the installation comprises a 49 mW HeCr laserproducing a monochromatic blue laser beam. The laser is associated witha scanning device (scanner) in an X-Y coordinate system associated witha clock pulse synchronization module (synchro). An optical system (Opt.I) x500-8000 is interposed between the scanner and the integratedcircuit IC under test. Circuit IC is connected to a 12 V d.c. electricsupply and to a signal generator (SG) by a connecting base (So) on whichit is placed during tests. The signals are chosen by a programmableselector (PS) controlled by a 16-bit microprocessor (Mi) connected tothe synchro module.

Having described the irradiation components of the installation and thecomponents for electronically stimulating the circuit, we shall nowdescribe the components for detecting secondary radiation and analysingthe signals resulting from electric stimulation.

The polychromatic secondary emission resulting from irradiation ofcircuit IC is picked up by an optical system (Opt. II) x500-8000. Thepolychromatic radiation is then filtered at a wavelength greater than orequal to 0.480 λ in the filter module (Fi) which communicates with aphotomultiplier (PhM). The signal is then converted from analog todigital by an 8-bit module (ADC) which supplies the 1024×1024×8 bitimage storage module (ISM) connected to an address generator (ADG)associated with the syncrhonization module (synchro).

Electric signals obtained by electric stimulation by the signalgenerator (SG) are amplified between 20 and 4000 times by an amplifier(A) connected to an 8-bit analog-digital converter (ADC II) via aselector (Se). The signals from the converter (ADC II) are transmittedto the microprocessor (Mi) and thence to a software unit (SOFT) foranalysis together with the processed image coming from animage-comparing logic operation system (XOR).

When the image has been stored in the storage module (ISM) it isprocessed by system (XOR), which is connected to a reference image store(RIM) and to a buffer store (PM) which in turn is connected to the (ISM)module by an address decoder (AD). The output of system (XOR) transmitsdetected differences or parts of pre-treated images to the software(SOFT), the processing program of which will be explained hereinafter.The computer then sequentially transfers the image processed by thecomparison system (XOR) to the microprocessor image.

The computer program comprises four sequences illustrated by rectangles(a), (b), (c), (d). (a) corresponds to location of faults in theprocessed image, (b) corresponds to a verification (based on a set ofsymbolic rules in accordance with the principles of artificalintelligence) of electric faults and faults in the image, (c)corresponds to detection, location and dimensioning of faults and (d) toan instruction for the next electric stimulation test.

In a variant, a first detector 3 can be an infrared scanning detector oran infrared detector comprising an array or row made up of a mosaic oftransducers, adapted to detect hot spots, e.g. of about 50° to 60° C.produced by electric excitation and having a spectrum band width in therange from 3 to 7 μm. A second infrared detector (3) having a spectrumband width in the range from 10 to 20 μm is adapted to detecttemperature gradients from 0.2° to 0.5° C. for detecting and measuringlocal intensities in metal coatings and in the conductive connections.

The radiation also induces an electric current which is superimposed onthe excitation signal from source 4 and can also reveal discontinuitiesin the structure. This is one reason why it is proposed to combine theelectric response signals from device 1 and the signals from detectors 3in module 8, the combination being used to evaluate structural defectsin device 1. This significantly reduces the duration of the examination.It is also possible to examine deviations due to the image, electricstimulation, or to a combination of these signals. Of course, the imagesmay also be used by an operator reponsible for making such a comparison.

I claim:
 1. A method of examining and testing a layered electric circuit device having signal processing capability at discontinuities contained internally of its layers of structure in which radiation emitted by the device itself, after stimulation, is detected by scanning, electric signals characteristic of the detected radiation are formed and the signals are compared with reference signals, said method being characterized in that:said stimulation of the device comprises, in combination, sending a given electric signal through the device for signal processing therewithin and simultaneously irradiating the device with penetrating radiation from at least one source of radiation of a predetermined type and having predetermined characteristics, the said radiation emitted by the device being the result of interaction of the applied irradiation with the material of the device structure through which the given electric signal travels at places where the structure has discontinuities.
 2. A method according to claim 1 characterized in that the radiation emitted by the device is also compared with the expected radiation to be emitted in response to said given stimulating electric signal travelling through the device.
 3. A method according to claim 1 characterized in that the irradiation source comprises a pulsed ion laser.
 4. A method according to claim 1, characterized in that the irradiation source comprises an infrared source (2-50 microns).
 5. A method according to claim 1 characterized in that the irradiation source comprises a millimeter microwave source.
 6. A method according to claim 1 characterized in that the irradiation source comprises a proton source.
 7. A method according to claim 1 characterized in that the irradiation source comprises a beam of pulsed electrons.
 8. A method according to claim 1 characterized in that the irradiation source comprises a neutron source.
 9. A method according to claim 1 characterized in that the irradiation source comprises an ion source.
 10. A method according to claim 1, characterized in that the irradiation source comprises an X-ray or UV laser.
 11. A method according to claim 1 characterized in that the irradiation source comprises a nuclear magnetic resonance (NMR) source.
 12. A method according to claim 1 characterized in that the irradiated surface of the device is covered with a liquid substance for locally amplifying the radiation emitted by the surface.
 13. A method according to claim 12 characterized in that the substance is a liquid gas.
 14. A method according to claim 12 characterized that the substance is made up of liquid crystals.
 15. A method according to claim 1 characterized in that at least one of the chemical, electromagnetic and temperature parameters of the device under test is controlled to have a predetermined value.
 16. A method according to claim 1 or 2 characterized in that the electric response signals of the electric device are combined in order to compare them with reference signals.
 17. A method according to claim 1 characterized by detecting radiation reflected by the device itself after being stimulated.
 18. A method according to claim 1 characterized by detecting radiation transmitted through the device itself after stimulation.
 19. A method according to claim 4 or 5 characterized by detecting the radiation emitted by the device itself after being stimulated in the millimeter wave region from 20 to 600 GHZ, by using a cadmium/mercury telluride monocrystal photoconduction detector in the hot electron mode.
 20. A method for examining and testing an electrical signal processing element having a layered structure of material, said method comprising the steps of:passing through the said element for processing a predetermined electrical signal; simultaneously therewith irradiating said element with radiation from at least one radiation source of predetermined type and known characteristics, said irradiation and said electrical signal acting on said element in combination to stimulate said element; said irradiation interacting with the material of the structure of said element through which said signal is passing at discontinuities present within said structure; scanning the said element and detecting radiation emanating from said element owing to the said combined stimulation; forming electrical response signals characteristic of said detected radiation; and comparing said electrical response signals with predetermined reference signals.
 21. A method according to claim 20, further comprising the steps of: (1) detecting the processed electrical response signals of the said element to the said stimulating electrical signal travelling through the device, and (2) comparing the said response signals of said element with the expected response to said stimulating electrical signal.
 22. A method according to claim 20 characterized by combining the electric response signals of the said element in order to compare them with the reference signals.
 23. A method according to claim 20 characterized in that the said detected radiation is radiation emitted by reflection by the said element after being stimulated.
 24. A method according to claim 20 characterized in that the said detected radiation is radiation emitted by transmission by the said element after said stimulation.
 25. A method according to claim 20 characterized by stimulating the said element by means of irradiation in the millimeter wave region from 20 to 600 GHZ, and detecting the radiation emitted by said element by a cadmium/mercury telluride mono-crystal photoconduction detector in the hot electron mode.
 26. A method of examining and testing an integrated, layered electric element having signal processing capability when excited by a supply voltage and suitable input electrical signals, said method comprising the steps of:applying a supply voltage and a predetermined electrical signal to said element and to a comparison module; simutaneously irradiating said element with penetrating radiation from at least one radiation source; detecting secondary radiation emitted by said element and radiation reflected by said element in response to said simultaneous irradiating radiation and electrical excitation of said element; converting said detected radiation into a stored memory representation characteristic of said electrical element under test; detecting any faults in said stored memory representation; comparing an output electrical signal from said element with a reference signal which corresponds to the output electrical signal that said element would produce if said element had no structural defects; using the information from said comparing step and from said detecting faults step to determine the extent to which said device corresponds to preset standards.
 27. A method as in claim 26 wherein the combination of said radiation and said electrical signal stimulating said element causes said radiation to interact with the material of said element through which said signal is passing at the discontinuities present in the structure of said element.
 28. A method as in claim 27 wherein a further layer is deposited on said element in order to enhance certain secondary radiation emitted. 